Jökull - 01.12.1976, Side 19

Sandvatn in the western volcanic zone, but
earthquakes were also recorded from the Reykja-
nes Peninsula, the volcanoes Surtsey and Katla,
and the area NE and E of Hekla (Fig. 1). Three
earthquakes from the last area could be ap-
proximately located (Fig. 1, Table 2).
TABLE2
Date Time Epicenter Magnitude
June 28 1159 64°10'N 19°15'W 3 (approx.)
July 2 0840 64°10'N 19°00'W 2i/2 (approx.)
Aug. 2 0813 64°00'N 19°10'W 4.8
All three earthquakes occurred close to a pro-
file of distance measurement lines established
in 1967. A remeasurement of these lines in Sept-
ember 1970 showed significant lengthening of
several lines on the profile (Decker et al., 1971).
The line that showed the maximum lengthen-
ing (21 mm) is situated near the epicenters of
the earthquakes of July 2 and August 2.
N
lOoh
10
I - • .
l—i----------------1_____________i______________I
3.0 3.5 4.0 4.5 M
Fig. 7. Cumulative number N(M) versus magni-
tude relationship for the earthquakes near Hekla
between May 6 and July 14, 1970 as recorded
by the AKU seismic station. The relationship
becomes nonlinear for M < 3.4. The maximum
likelihood estimate of the b-value for M =2 3.4
is 1.6 ± 0.3.
Mynd 7. Fjöldi skjálfta og stœrð þeirra á tima-
bilinu 6. mai til 14. júli samkvœmt mali á Aliur-
eyri. Mœlirinn nœr ekki að skrá alla skjálfta,
sem eru minni en M 3,4. Skjálftar stærri en það
hafa b-gildi 1,6 ± 0/.
The earthquake on June 28 occurred near
the NE end of a fissure swarm that runs through
Hekla from SW to NE. An experimental water
reservoir is located in this area. In June 1970
a sudden drop of 2.5 m was observed in the
reservoir level and in the spring of 1971 the
reservoir drained itself through a fissure system
that opened up on its bottom (Tómasson, 1971).
Fissures possibly started to open in June 1970
causing the drop in the reservoir level. The
June 28 earthquake might have been associated
with tectonic activity near the reservoir.
CONCLUSIONS AND DISCUSSION
1. Irregularities in the P-wave travel times
on tlie northwest side of Hekla can be explained
by a dip of the crustal layers above layer 3.
The most probable dip is 10° to the SE. This
conclusion is consistent with the results of re-
fraction studies by Pálmason (1971) that indi-
cate a dip in the layer 2—layer 3 boundary of
at least 5° to the SE in this general region.
2. The beginning of the 1970 eruption was
preceded by continuous volcanic tremors. The
tremors were recorded by three permanent seis-
mograph stations in Iceland for 25 minutes be-
fore the eruption broke out. It is likely that a
sensitive short-period seismograph if located in
the immediate vicinity of the volcano would
have recorded tremors for several hours before
volcanic activity became visible at the surface,
thus giving a warning of an impending erup-
tion.
It is not within the scope of this paper to
evaluate the importance of being able to pre-
dict eruptions of Hekla. But if such predictions
are considered to be valuable it is important
that sensitive short-period seismometers be
operated permanently in the immediate vicinity
of the volcano.
3. The seismic activity during the eruption
occurred mainly within three periods of time
separated by periods of low seismicity. The first
active period was coincident with the beginn-
ing of the eruption and subsequent changes
when the volcanic activity ceased at three dif-
ferent eruption sites. The second active period
was not related to any obvious change in the
eruption. The third period of seismic activity
was clearly related to the end of the volcanic
JÖKULL 26. ÁR 1 7